Considerable progress has been made in this laboratory in defining and identifying proteins associated with mRNA in L-cell polyribosomes with the aid of ultraviolet light-induced crosslinking of proteins to mRNA. Also, it has been found that these proteins are synthesized and become associated with mRNA in the absence of mRNA synthesis. This observation suggests that the proteins function in polyribosomes, and it is consistent with the idea that they have a role in protein synthesis. It is proposed to continue and extend these studies. In particular, additional work on the enumeration and identification of proteins associated with poly (A) and with sequences other than poly (A) in mRNA will be done. Also, it will be determined whether or not the proteins that can be crosslinked to mRNA interact with ribosomes as well as with mRNA. In related work the proteins associated with heat shock mRNAs and with 25 degrees C mRNAs in Drosophila cells will be investigated to determine if mRNA-associated proteins might be involved in control of translation. Also, the phosphorylation of these proteins in heat shock and in normal conditions will be investigated. Finally, attempts will be made to identify proteins associated with a particular kind of mRNA (Drosophila 70K heat shock mRNA) and with particular regions of the mRNA molecule. These studies will utilize ultraviolet light-induced crosslinking of proteins to RNA and related methodology recently developed in this laboratory.